The present invention relates generally to an overcharge preventing device for secondary batteries, and more particularly to an overcharge preventing device for a specific secondary battery of high energy density, e.g., a lithium ion battery using a lithium ion doping or de-doping material as an active negative electrode material.
A lithium ion battery using a lithium ion doping or de-doping carbonaceous material can output high voltage and has high energy density albeit being light in weight, and so is now widely used on portable equipment. Although secondary batteries may be used over and over by charging, yet their performance may become worse upon charging. Especially in the case of a lithium ion battery, various safety devices are incorporated therein so as to prevent overcharging which may otherwise cause an electrolyte leakage due to the actuation of a pressure valve due to gas generation, resulting in thermal runaway, and fuming.
The lithium ion battery is charged in a constant voltage, constant current charging mode to prevent overcharging. Insofar as the charging voltage of a charger is kept constant, overcharging is unlikely to occur.
However, this is not true of the case where the charger fails, or the lithium battery is charged without recourse to a dedicated charger, e.g., using a charger of high charging voltage; the lithium ion battery is overcharged.
To avoid this problem, the inventors have filed Japanese Patent Application No. 286757/1986 to come up with a secondary battery protector comprising a battery voltage sensor and a switch for cutting off a charging current by the voltage sensor, so that upon detection of overcharge, the charging current is cut off to interrupt charging.
The present invention is directed to a protector for a secondary battery having high energy density such as a lithium ion battery using a lithium ion doping or de-doping carbonaceous material as an active negative electrode material. A particular object of the invention is to provide a secondary battery protector which, upon overcharging, is actuated to interrupt charging and, upon elimination of the overcharged state, is actuated to recharge the battery.